The use of diagnostic ultrasound to examine the interior portions of a living creature without exploratory surgery is known. Ultrasound examination is preferred over exploratory surgery for obvious reasons. Exploratory surgery requires anesthesia, cutting of tissue and exposure of a body portion. These procedures carry the inherent risk of anesthesia and also require inflicting local tissue trauma with accompanying inflamation and introducing the risk of infection. Both local tissue trauma and infection may cause secondary effects throughout the body. In contrast, ultrasound is non-invasive, so it does not require anesthesia and causes no trauma or risk of infection. Avoiding exploratory surgery with its accompanying consequences is desirable in any clinical setting and is essential if the response of blood vessels or any other tissue to specific traumatic conditions are to be measured.
The use of ultrasound to examine arteries is known. An early ultrasound instrument utilized the A-mode of display to measure the external diameter of the exposed aorta and femoral artery in an animal model. When the arteries were exposed and the crystal was placed directly over the artery, it was possible to identify the structures (walls) giving rise to the echoes displayed in the A-mode. See Hokanson, D.E., et al., A phase locked echo tracking system for recording arterial diameter changes in vivo, J. Appl. Physiol. 32:728-735, 1972. However, it was difficult to identify the structures from which the echoes originated without exposing the vessels because, the skin, muscle and other tissues located between a crystal from which ultrasonic radiation is generated and the artery can give rise to independent echoes of their own. For this reason the development of ultrasound equipment for use in examining blood vessels has taken a rather different direction. The equipment most frequently employed utilizes real time, B-mode, scanning (continuous gray scale). Real time means an instantaneous representation of the section being examined such that it instantly shows changes in the structure being examined. The B-mode provides a recognizable two dimensional image of structures displayed on a video screen. See generally Kremkau, Diagnostic Ultrasound: Physical Principles and Exercises, Grunne and Stratton, 1980. Real time B-mode ultrasound has found wide use in examination of extracranial carotid arteries for atherosclerotic lesions. Other arteries and veins have also been examined by ultrasound. In these studies real time B-mode has been used and measurement of vessel diameter has often been made from frozen images on the viewing screen. The following textbooks generally describe the principles and uses of the diagnostic ultrasound and provide a background for understanding the present invention: Goldberg, Kolter, Ziskin and Waxham, Diagnostic Uses of Ultrasound, Grune and Stratton, 1975; Wells, P.N.T., Biomedical Ultrasonics, Academic Press, 1977; McDicken, Diagnostic Ultrasonics: Principles and Use of Instruments, Second edition, Wiley Medical, 1981; Wells, P.N.T. and Ziskin, M.C., New Techniques and Instrumentation in Ultrasonography, Churchill Livingston, 1980; Sanders, R. and James. S.A., Ultrasonography in Obstetrics and Gynecology, Appleton-Century-Crofts, 1980; Berstein, E.F. Editor, Noninvasive Techniques in Vascular Disease, C.V. Mosby Co., 1979; and Repacholi, H. and Benwell, D.A., Essentials of Medical Ultrasound: A Practical Introduction to The Principles, Techniques and Biomedical Applications, Humana Press, 1982.
The following publications are also illustrative of the state of the art in diagnostic ultrasonic techniques. Reference to these publications is suggested for a greater understanding of the invention: Hokanson, D. E., et al., A Phase Locked Echo Tracking System for Recording Arterial Diameter Changes In Vivo, Journal of Applied Physiology, Vol. 32, No. 5, May 1972, pp. 728-733; Olson, R.M., et al., A Nondestructive Technique to Measure Wall Displacement In The Thoracio Aorta, Journal of Applied Physiology, Vol. 32, No. 1, January 1972, pp. 147-151; Garth, K. E., et al., Duplex Ultrasound Scanning of The Carotoid Arteries With Velocity Spectrum Analysis, Radiology, Vol. 147, No. 3, June 1983, pp. 823-827; Evans G. C., et al., Echoaortography, The American Journal of Cardiology, Vol. 19, January 1967, pp. 91-96; James, E. M., et al., High Resolution Dynamic ULtrasound Imaging Of The Carotid Bifurcation: A Prospective Evaluation, Radiology, Vol. 144, September 1982, pp. 853- 858; and Comerota, A. J., et al., Real-Time B-Mode Carotid Imaging In Diagnosis of Cerebrovascular Disease, Surgery, Vol. 89, No. 6, June 1981, pp. 718-729.
In general, real time ultrasound provides a means of studying the response of blood vessels in situ without surgically exposing the vessel. This is important for two reasons. First, vascoactive substances released or synthesized in response to the surgery required to expose the vessel may very well induce constriction or dilation of the vessel. These changes in vessel diameter make it impossible to obtain true data on the response of the vessel to experimental factors. Second, surgical exposure of a blood vessel in the human patient is not acceptable, so a noninvasive method is required for any human study. Angiography cannot be used for prolonged or repeated measurements and in addition the radiopaque material may cause a change in vessel diameter immediately.